1. Field
This invention relates to the field of video signal processing. More specifically, a video processing system and method is described in which the contrast of a video signal is adaptively enhanced while the temporal and spatial visual effects in the original video signal are preserved.
2. Related Art
The contrast of a picture is a property related to the luminance variations of the picture. Some pictures may have low contrast due to, for example, poor lighting conditions in the capturing process or a limited dynamic range of the image sensor used in the capturing process. It is well known that the human visual system is more sensitive to the contrast of a picture than the absolute luminance values. Therefore, contrast enhancement is often a necessary technique in many image processing applications.
Video can be thought of as a sequence of pictures captured at a certain picture rate. For example, NTSC video is at 60 fields/s, PAL video is at 50 fields/s, and ATSC 720 p video can be at 60 frames/s, etc. Contrast enhancement for video shares some common requirements with still picture contrast enhancement. For example, the purpose of both is to increase the visibility of the details in low-contrast areas, and the intensity ordering of neighboring pixels should be kept intact (i.e., if an original pixel X has a higher intensity level than one of its neighboring original pixels Y, then the contrast-enhanced enhanced pixel X′ should have an intensity level not lower than the contrast-enhanced pixel Y′). On the other hand, due to the time-varying property of video, video contrast enhancement has different requirements from picture contrast enhancement. An important difference is that contrast variations, which commonly exist in video signals as the artistic intention of the producer, should be preserved after contrast enhancement. Two known examples of contrast variations in video are: (1) fade—where a video sequence gradually changes from one scene to another scene, often black or white, and (2) flash—where a sequence contains sudden intensity changes. Another important difference is that video contrast enhancement may need to be performed in real-time, i.e., at the same speed as the picture rate of the video signal. Due to these differences, directly applying still picture contrast enhancement methods to video signals often creates unrealistic and noticeable artifacts.
Therefore, a method of video contrast enhancement that enhances the contrast of a video signal while preserving the original visual effects is needed.